To maintain the integrity of pipelines, it is known to employ inspection vehicles that travel internally to the pipe recording information about the quality of the pipe wall. The majority of these inspection vehicles use ultrasonic or magnetic sensors to carry out the inspection.
The preferred technology for crack detection is ultrasonic inspection, and in pipelines carrying liquids a vehicle-based inspection system can use the liquid as a coupling medium to assist conventional piezo-electric sensors. These sensors are capable of carrying out well-established inspection techniques leading to high resolution of defects and discrimination of defects based on through-wall positional resolution. In gas filled pipelines it is difficult to use conventional piezo-electric transducers because the gaseous pipeline product is not an efficient coupling medium for introducing ultrasound into the pipe wall.
An alternative type of sensor, which does not rely on liquid coupling, is an electromagnetic acoustic transducer (EMAT).
EMATs are dry-coupled. They generate sound directly in the test material by one of two main mechanisms. A first mechanism involves Lorentz forces, a second mechanism involves magnetostrictive forces. For transmission into the pipe wall according to the first mechanism, an alternating current in a wire induces an eddy current in the metal surface. When this is combined with a static magnetic field, a Lorentz force may be produced that causes the “grid” of metal in the pipe wall to oscillate, launching ultrasonic sound waves in the pipe wall. Breaks in the homogeneity of this metal grid (e.g. defects such as cracks) will result in reflections of the sound wave. These reflected waves encountering the magnetic field will generate an eddy current, which in turn, induces a current in the wire. This current forms the received signal, which can be further processed and analyzed. EMATs transmitting according to the second mechanism rely on magnetostrictive forces that result from the AC magnetic field. These EMATs may receive sound an convert it to an electrical signal by the inverse magnetostrictive effect.
EMATs can be used to perform ultrasonic inspection of gas pipelines. However, the spatial resolution of the inspection is usually lower than for conventional (e.g. piezo-electric) sensors operating in pipelines which carry liquid. A consequence of this is that EMAT sensors, which are able to detect reliably the presence of pipe wall defects, are comparatively poor at providing sufficient information to identify the type of pipe wall defect.